Digital exposure meter

ABSTRACT

This invention relates to a digital exposure meter in which brightness information is digitized and exposure factors such as film sensitivity are converted to digital values to perform digital computation. Accordingly, an exposure value, average exposure value, aperture value or shutter time is derived while a computer having various operation functions being controlled in sequence by sequence control means.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to exposure meters for use with photographic cameras, and, more particularly, to a digital exposure meter in which analog quantities are converted to corresponding digital quantities based on which a proper exposure value is derived by computation.

2. Description of the Prior Art

The prior art provides many methods of deriving exposure values for use in photography, most of which may be grouped into the following three types:

(1) Derivation of an effective exposure aperture for the photographic lens as a result of computation from the object brightness, film sensitivity and shutter speed;

(2) Derivation of an effective exposure time as a result of computation from the object brightness, film sensitivity and aperture value of the photographic lens;

(3) Derivation of an exposure value for a given photographic situation as a result of computation from the object brightness and film sensitivity.

All of these types have respective characteristics and advantages, and have so far been put into practice, but with some drawbacks.

The method of type (1) described above, because of its possibility of preselection of a desired shutter speed, permits the photographer who encounters a given photographic situation associated with a high possibility of introducing vibrations into the hand-held camera, when snap photography is to be made, or when an object which is moving at a high speed is included therein, or when a telephoto objective lens is used with the camera, to take into account the movement of the object and the magnitude of the vibrations. This is done so as to select a relatively faster shutter speed but does not enable him to observe and evaluate in detail the far limit and the near limit of the depth of the total scene prior to making an exposure, because the effective exposure aperture is automatically determined with the preselected shutter speed. Therefore, this first type is not suitable for use in taking a picture of landscape or still life of which the quality is largely dependent upon adjustment of the depth of field, or taking a vignette by making use of the controlled depth of field.

The method of type (2) described above operates in an entirely opposite sense to that of the type (1) method, permitting for the photographer who is about to take a picture of landscape or still life, or to take a vignette, to adjust the size of diaphragm aperture by taking into account the depth of field, but will result in a reduction of the percentage of photographs taken with relatively long shutter speeds which will be found acceptable particularly when the given photographic situation includes an object which is moving at a relatively high speed, or when the camera is hand-held. Therefore, this second type is not suitable for use in taking a picture of an object being moved at a fast speed, or taking picture while the camera is being held by hand.

The method of type (3) described above provides an exposure value which when used in making an actual exposure must be factored into effective exposure time and aperture based on his personal experience and insight.

Accordingly, it is desirable to provide an exposure meter capable of selectively performing either of the methods of types (1) and (2) by the free choice of the photographer as the controls of shutter speed and aperture value have different imaging effects from each other. With this in mind, an analog exposure meter has been proposed which is provided with a function of deriving an effective exposure aperture in automatic response to the sensitivity of the used film, the preselected shutter speed and the level of brightness of an object being photographed, and with a function of deriving an effective exposure time in automatic response to the sensitivity of the used film, the preselected aperture and the object brightness level. This analog exposure meter necessitates an electrical circuit for processing photographic information in the analog form to compute an exposure value, thereby giving a disadvantage of making it practically impossible to manufacture a production run of circuit elements thereof economically while holding all of the circuit parameters to specific and ideal values.

In connection with the method of type (3), there are two metering modes, namely, a spot-metering mode and an overall field of view light metering mode. The spot-metering mode when applied to an exposure determination for that portion of a scene being photographed which constitutes a subject of principal interest provides a correct exposure as the environment of the subject is of no importance. But, when the area of the image within the spot of the subject of principal interest is larger than the area of the spot based on which the exposure determination is made, it is generally impossible to make a correct exposure for the entire area of the subject. The overall field of view light metering mode works in an entirely opposite sense to that of the spot-metering mode. It is known in the art to provide a camera of which light metering aspects may be selectively operated in a spot-metering mode or overall field of view light metering mode. In this case, it is also necessary to process photographic information in the analog form to compute an exposure value. Accordingly, likewise as above, the computing circuit is made complicated in structure, and a requirement for high accuracy of light metering is not satisfied.

SUMMARY OF THE INVENTION

Accordingly, the present invention has, for the general object, to provide an exposure meter for a photographic camera of which exposure value deriving aspects may be selectively operated in a shutter speed preselection automatic aperture value deriving mode, or aperture value preselection automatic shutter time deriving mode, and of which light metering aspects may be selectively operated in a spot-metering mode or overall field of view light metering mode.

An object of the invention is to provide an exposure meter which is constructed from a digital computer of which various electrical circuits may be fabricated in an integrated circuit form and which has overcome the above mentioned conventional drawbacks thereby.

Another object of the invention is to provide a digital exposure meter capable of computing an average exposure value from a number of particular exposure values based on corresponding portions of the area of an image of the total scene.

Another object of the invention is to provide a digital exposure meter which permits for a photographer to make an exposure determination with a desired shutter speed or aperture value based on the portion of the image within a particular spot or on the total image selectively.

Other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view, partially in block form, of the circuitry of a digital exposure meter according to the present invention.

FIG. 2 is a block diagram of a ROM device usuable in the digital exposure meter of FIG. 1.

FIG. 3 is a top view of the digital exposure meter of FIG. 1 having a number of keys.

FIG. 4 is a block diagram showing a program incorporated in the ROM device of FIG. 2 for light measurement.

FIG. 5 is a block diagram showing a program incorporated in the ROM device of FIG. 2 for average light measurement.

FIG. 6 is a block diagram showing a program incorporated in the ROM device of FIG. 2 for shutter time computation.

FIG. 7 is a schematic circuit diagram, partially in block form, of a discriminating circuit usuable in the circuit of FIG. 1.

FIG. 8 is a schematic circuit diagram, partially in block form, of an exponent circuit usuable in the circuit of FIG. 1.

FIG. 9 is a block diagram showing a program incorporated in the ROM device of FIG. 2 for aperture value computation.

FIG. 10 is a schematic circuit diagram, in partially in block form, of an adder 2 of FIG. 1.

FIG. 11 is a schematic circuit diagram of a film sensitivity setter.

FIG. 12 is a schematic circuit diagram of a reciprocal number circuit 6 of FIG. 1.

FIG. 13 is a schematic circuit diagram of a shift register usuable in the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown one embodiment of a digital exposure meter according to the present invention as comprising a sensor circuit having a photosensitive element SPC connected between two inputs terminals of an operational amplifier OP₁ and having a log diode D₁ connected in the feedback network of operational amplifier OP₁, a comparator OP₂ having two input terminals one of which is connected to the output terminal of amplifier OP₁ and the other of which is connected to an output terminal of a digital-to-analog converter DA and having an output connected to a "reset" input terminal R of a clock pulse generator PG, a latch circuit L connected between digital-to-analog converter DA and a first shift register AR having an input terminal connected to a common output terminal of clock pulse generator PG through an AND gate G₁, second and third shift registers BR and CR, the second shift register BR having an input terminal connected both to the common output terminal of generator PG through an AND gate G₂ and to an output terminal of a computing circuit AC through an AND gate G₂₁, and the third shift register CR having an input terminal connected through an AND gate G₃ to the common output terminal of clock pulse generator PG, a first memory register M1R having two parts RA and RN with the input terminals thereof being connected through respective AND gates G₅ and G₄ to the common output terminal of clock pulse generator PG, second and third memory registers M4R and M3R having input terminals connected through respective AND gates G₆ and G₇ to a control parameter input circuit CO associated with a key board of FIG. 3, a display device having a decoder DE made of a diode matrix and having a number of display elements such as seven segments DS and having a reciprocal number circuit 6, the decoder DE having an input terminal connected both to the output terminal of first shift register AR through an AND gate G₃₃ and to the output terminal of reciprocal number circuit 6, and the reciprocal number circuit 6 having an input terminal connected through an AND gate G₃₄ to the output terminal of first shift register AR, a film speed Sv having an output terminal connected through gate G₁ to the input terminal of first shift register AR, and a discriminating circuit I having two input terminals one of which is connected through an AND gate G₁₉ to the output terminal of first shift register AR and the other of which is connected through an AND gate G₁₈ to an output terminal of part RN of first memory register M1R, and having an output terminal connected both to the gating control input terminal of gate G₃₃ through an AND gate G₂₅ and to an input terminal of a read-only-memory device ROM through an AND gate G₂₄.

The computing circuit AC comprises a square-root circuit 1 having an input terminal connected through an AND gate G₈ to the output of first shift register AR, an adder 2 having two input terminals one of which is connected through an AND gate G₉ to the output terminal of first shift register AR, and the other of which is connected to the output terminals of third shift register CR, parts RA and RN of first memory register M1R through respective AND gates G₁₁, G₁₀ and G₁₂, a divider 3 having two input terminals connected to the respective output terminals of second and third shift registers BR and CR through respective AND gates G₁₃ and G₁₄, an exponent circuit 4 having an input terminal connected through an AND gate G₁₅ to the output terminal of first shift register AR, and a multiplier 5 having two input terminals connected to the respective output terminals of second and third shift registers BR and CR through respective AND gates G₁₇ and G₁₆, the parts 1 through 5 having a common output terminal connected to the input terminal of first shift register AR through an AND gate G₂₀, and to the output terminal of second shift register BR through AND gate G₂₁ and to the input terminal of part RN of first memory register M1R through an AND gate G₂₃.

The output terminals of first, second and third shift registers AR, BR and CR and two parts RA and RN of first memory register M1R are connected through respective inhibit AND gates G₂₆ through G₃₀ to both of the input terminals of AND gates G₁ and G₅ and therefrom to the input terminal of first shift register AR and to the part RA of first memory register M1R respectively. First shift register AR is further provided with an AND gate G₃₄ connected between the input and output terminals thereof and with a shift register R₁ having an output terminal connected to the input terminal of register AR, and having an AND gate G₁ ' connected between the input and output terminals of register R₁. The third shift register CR is further provided with an AND gate G₃₂ connected between the input and output terminals thereof. The discriminating circuit I is further provided with a shift register R₂ having an AND gate G₂ ' connected between the input and output terminals of register R₂ and having an output terminal connected through to the respective input terminals of discriminating circuit I through AND gates G₃ ' and G₄ '. The output terminal of discriminating circuit is connected through AND gate G₂₅ and an inverter IN to the gating control input terminal of AND gate G₃₄ having an output terminal connected to the input terminal of reciprocal number circuit 6.

The gating control input terminals of all of the gates G₁ through G₃₂ and G₁ ' through G₄ ' are connected to output terminals of the ROM device of FIG. 2, so that a number of control signals are sequentially applied from the ROM device to those of the gating control input terminals which are selected according to the selected one of the programs illustrated in FIGS. 4, 5, 6 and 9. The selection of the programs and the introduction of exposure control parameter values into the exposure meter can be effected on the key board of FIG. 3, in which keys designated by digit characters 0 through 9 and point are adapted for introducing a value of a desired shutter speed, or aperture value, keys designated by T and 1/T for setting the shutter speed, a key designated by F for setting the aperture value, a key designated by Ev for selecting the light measurement program of FIG. 4, a key designated by Ev for selecting the average light measurement program of FIG. 5, a key designated by A for selecting either the shutter time computation program of FIG. 6 or the aperture value computation program of FIG. 9, and a digital switch designated by DIN for setting the film speed, and DS designates the read-out device of FIG. 1.

The operation of the digital exposure meter of FIGS. 1, 2 and 3 will be described in connection with FIGS. 4, 5, 6 and 9. For making an exposure value determination based on the program of FIG. 4, the operator may push down Ev key, thereby the program address assigning circuit 21 of FIG. 2 is actuated causing the program memory 22 to render the program of FIG. 4 cooperative with the instruction register 23 which in turn causes generation of eleven pulses E₁ through E₁₁ in sequence through the instruction decoder 24 and instruction generator 25.

First signal E₁ is applied to first shift register AR, clock pulse generator PG, gates G₁ and G₂₆, thereby register AR and generator PG are rendered operative and gates G₁ and G₂₆ are gated on and off respectively. As generator PG produces a clock pulse train which is passed through gate G₁ to register AR, the register AR counts the number of pulses produced and stores in the form of a time variable binary code, that is, the binary condition "1" or "0" in each of the bits of register AR. The number of pulses counted by register AR is converted to a corresponding voltage by digital-to-analog converter DA having a ladder type resistor network known in the art. When the output voltage of converter DA has reached a level equal to the output voltage of the sensor which is proportional to the logarithm of the object brightness level, comparator OP₂ produces an output signal which is applied to the "reset" input of clock pulse generator PG, thereby generator PG is rendered inoperative. During the time interval the clock pulse train enters register AR, gate G₂₆ is gated off to prevent the output of register AR from exiting therefrom to the other registers. Second signal E₂ is applied to first and third shift registers AR and CR and gates G₃ and G₂₈, causing the content of first register AR to be shifted and to be transferred through gate G₂₆ and gated-on gate G₃ to third shift register CR. Third signal E₃ is applied to first register AR and gates G₁ and G₂₆, causing a digital output of film speed setter Sv to be transmitted into first register AR through gate G₁. Fourth signal E₄ is applied to first and third shift registers AR and CR and gates G₂₆, G₂₈, G₉, G₁₁ and G₂₀, causing the outputs of first and third shift registers AR and CR to be transmitted through gates G₉ and G₁₁ to adder 2 and causing the output of adder 2 which is representative of Bv+Sv to be transmitted through gate G₂₀ to first shift register AR. During this time interval, gates G₂₆ and G₂₈ are gated off to insure that the outputs of first and second shift registers AR and CR are applied to only adder 2. Fifth signal E₅ is applied to first and second shift registers AR and BR and gates G₂, G₂₇ and G₃₁, causing the AR content Bv+Sv=Ev to be transferred to second shift register BR through gates G₂₆ and G₂, and causing AR content to be again introduced to first register AR through gate G₃₁. Sixth signal E₆ is applied to RA part of first memory register M1R, first shift register AR, and gates G₂₆, G₂₉, G₁₀, G₉ and G₂₀, causing the output of RA part of first memory register M1R and first shift register AR to be applied to adder 2 through gates G₉ and G₁₀ and causing the output of adder 2 to be applied to first shift register AR through gate G₂₀. Seventh signal E₇ is applied to first shift register AR, RA part of first memory register M1R and gates G₅ and G₂₉, causing the content of first shift register AR to be transferred through gate G₅ to RA part of first memory register M1R. As a result from signals E₁ through E₇, the value Ev(=Bv+Sv) of first shift register AR is added to the sum of a number of values Ev' s previously stored in RA part of first memory register M1R and equal to the number of light measurements repeated. It is not assumed that the number of light measurements repeated is zero, in other words, this light measurement is of the first time. Eighth signal E₈ is applied to register R₁ which stores a digital value corresponding to "1" of first shift register AR, and gates G₂₆ and G₁ ', causing the content of register R₁ to be transferred to first shift register AR, and causing the content of register R₁ to be again introduced thereto through gate G₁ '. Ninth signal E₉ is applied to first shift register AR, RN part of first memory register M1R, and gates G₉, G₁₂, G₂₃, G₂₆ and G₃₀, causing the outputs of first shift register AR and RN parts to be applied through gates G₉ and G₁₂ to adder 2, and causing the output of adder 2 to be applied to RN part through gate G₂₃. As a result from signals E₈ and E₉, the number of light measurements repeated and stored in RN part of first memory register M1R is increased by one. But this light measurement is of the first time, so that the RN part has a binary condition "1". Tenth signal E₁₀ is applied to first and second shift registers AR and BR and gate G₁ and G₂₆, causing the content of second shift register BR to be transferred through gate G₁ to first shift register AR, that is, the transfer of value Ev from second to first shift register. Eleventh signal E₁₁ is applied to first shift register AR and gates G₂₆, G₃₁ and G₃₃ causing the value Ev of first shift register AR to be displayed by the display device DS through gate G₃₃ and decoder driver DE and causing the value Ev to be again introduced into AR.

For making an exposure value determination based on the program of FIG. 5, Ev key is pushed down to select the program of FIG. 5 for cooperation with the ROM device of FIG. 2. First signal E₁ is applied from ROM device to first shift register AR, RA part of first memory register M1R and gate G₁ and gate G₂₆, causing the content of RA to be transferred to AR through gate G₁. Second signal E₂ is applied to first and second shift registers AR and BR, gates G₂ and G₂₇, causing the content of AR to be transferred to BR through gate G₂. As a result from signals E₁ and E₂, the sum of values Ev' s stored in RA part is transferred to BR. Third signal E₃ is applied to first shift register AR, RN part of first memory register M1R and gates G₁ and G₂₆, causing the content of RN part to be transferred to AR through gate G₃₀ and G₁. Fourth signal E₄ is applied to first and third shift registers AR and CR and gates G₃ and G₂₈, causing the content of AR to be transferred through gates G₂₆ and G₃ to third register CR. As a result from signal E₃ and E₄, the number of light measurements repeated and stored in RN part is transferred to CR. Fifth signal E₅ is applied to first, second and third shift registers AR, BR and CR and gates G₁₃, G₁₄, G₂₀, G₂₆, G₂₇ and G₂₈, causing the outputs of second and third shift registers BR and CR to be applied through gates G₁₃ and G₁₄ to divider 3, and causing the output of divider 3 to be applied through gate G₂₀ to AR. The output of divider 3 is representative of an average value of the number of values Ev's. Sixth signal E₆ is applied to first shift register AR, RA part of first memory register M1R and gates G₅, G₂₉, G₃₁ and G₃₃, causing the content of AR to be transferred to RA part of M1R through gates G₂₆ and G₃, and causing the average value Ev to be displayed by display device DS through gate G₃₃ and decoder driver DE, and further causing the content of AR to be again introduced to AR, thereby the procedure of the average light measurement program of FIG. 5 is terminated.

For making an exposure time determination, a desired aperture value is set into the meter by pushing down certain of digit keys 0 through 9 and a decimal point key of FIG. 3, thereby the setting of the desired aperture value, A-value, is converted by input circuit CO known in the art to a corresponding digital value. Next key "F" is pushed down to apply a gating control signal to gate G₆, thereby the A-value is introduced into and stored in second memory register M4R. Next key "A" is pushed down to select the shutter time computation program of FIG. 6 for cooperation with ROM device. First signal S₁ is applied to RN part of first memory register M1R, register R₂ which stores a digital value corresponding to "1", and gates G₁₈, G₂₄, G₃₀, G₂ ' and G₃ ', causing the output of RN party of M1R and the output of register R₂ to be applied to discriminating circuit I. When the output of RN part is smaller than that of register R₂, in other words, when the number of light measurements repeated is zero, discriminating circuit I produces an output signal which is applied through gate G₂₄ to ROM device, thereby signals E₁ through E₄ are caused to occur. On the other hand, when discriminating circuit I produces no output signal, in other words, when at least one light measurement is already made, second signal S₂ is applied to first shift register AR and gate G₁₅ and exponent circuit 4, causing the Ev value of AR to be applied to exponent circuit 4. FIG. 7 is a circuit diagram showing construction of discriminating circuit I. Using FIG. 7, the operation of discriminating circuit will be explained in detail. In FIG. 7, RN designates RN part of first memory register M1R of FIG. 1, R₂ designates register R₂ of FIG. 1, G₁₈ designates gate G₁₈ of FIG. 1, IN₁ designates an inverter connected to RA part of first memory register M1R, AND₁ designates an AND circuit connected to inverter IN₁, the output of R₂ and the output terminal F₁ of T flip-flop TFF, IN₂ designates an inverter connected to the output terminal of R₂, AND₂ designates an AND circuit connected to inverter IN₂, RA part of first memory register M1R and the output terminal F₁ of T flip-flop TFF, OR designates an OR gate connected to the outputs of AND circuits AND₁ and AND₂ and having an output connected to terminal T of T flip-flop, and RSFF designates a RS flip-flop having a "set" input terminal connected to the output of AND₁ and having a "reset" terminal connected to output terminal of AND₂. Now assuming that the value of RN part is zero, signal S₁ causes the values of register R₂ and RN part of M1R to be read out and to be transmitted to inverters IN₁ and IN₂ and AND circuits AND₁ AND₂. The output of AND circuit AND₁ is "1", and the output of AND₂ is "0", so that F₂ is set to "1" and TFF is inverted by the output of OR, thereby F₁ becomes "0" to terminate comparison. (provided that TFF is in the state F₁ "0"; F₁ "1" by the reset pulse). As mentioned above, when no light measurement is previously made, F₂ of RSFF produces an output "1" which is applied to ROM device. When the content of RA part is more than "1", the state of RSFF remains unchanged, transmitting no signal to ROM device. As mentioned before, at the time when first signal S₁ occurs, the discrimination whether or not at least one light measurement is previously made is carried out. When no light measurement is made, the discriminating circuit I produces an output signal which is then applied through gate G₂₄ to ROM device, thereby signals E₁ through E₄ causes a light measurement of Ev value. Now assuming that the discriminating circuit I produces no signal, in other words, at least one light measurement is previously made, signals E₁ through E₄ do not occur from ROM device, and instead signals S₂ and S₃ occur to perform computation of 2^(Ev) from the value Ev of AR, which is transferred to second shift register BR. In other words, second signal S₂ is applied to first shift register AR and gate G₁₅ and gate G₂₆, causing the output of first shift register AR to be applied to exponent circuit 4. FIG. 8 is a circuit diagram showing exponent circuit 4, in which R₆ ', R₃, R₄ are shift registers, R₅ is a counter, Ex is an exclusive OR gate, NOR is a NOR gate, G₆ ' and AND are AND gates, IN is an inverter, and PG is a pulse generator. Second signal S₂ causes the content of AR to be transferred through gate G₁₅ to register R₄, thereby Ev value is introduced into register R₄. Next signal S₂₁ is applied to pulse generator PG, causing pulse generator PG to produce a pulse train which is applied to counter R₅ and AND gate AND. To other input terminal of AND gate AND is applied the output of NOR through inverter IN and the output of NOR is "0", so that a shift pulse is applied to register R₃ through AND, causing the content of R₃ to be shifted in correspondence to the number of pulses from pulse generator PG. When the content of R₅ coincides with the content of R₃, Ev number of shifts are effected. Also the digital value corresponding to "1" is transferred by second signal S₂ from R₆ to R₃, so that when the content of R₃ is shifted Ev number of times, computation 2^(Ev) is effected. Third signal E₃ is applied to register R₃ of FIG. 9, register BR of FIG. 1 and gates G₂₁ and G₂₇, causing the content of R₃ to be transferred through gate G₂₁ to BR. Fourth signal S₄ is applied to registers M4R and AR, and gates G₁ and G₂₆, causing the A-value of M4R to be transferred through gate G₁ to AR. Fifth signal S₅ is applied to registers AR and CR and gates G₃ and G₂₈, causing the content of AR to be transferred through gates G₂₆ and G₃ to register CR. Sixth signal S₆ is applied to registers AR, BR and CR and gates G₁₃, G₁₄, G₂₀, G₂₇, G₂₆, G₂₈ and G₃₂, causing the content of BR and the content of CR to be applied to divider 3, causing the output of divider which is representative of 2^(Ev) /A to be applied through G₂₀ to AR, and causing A-value of CR to be again transferred through gate G₃₂ to CR. Seventh signal S₇ is applied to AR, BR, G₂ and G₂₇, causing the content of AR to be transferred through gates G₂₆ and G₂ to BR. Eighth signal S₈ is applied to registers AR, BR and CR and gates G₁₃, G₁₄, G₂₀, G₂₆, G₂₇ and G₂₈, thereby computation based on the formula BR/CR= 2^(Ev) /A² = 1/T is performed and 1/T is transferred in a manner similar to that of signal S₆. Ninth signal S₉ is applied to registers AR and R₂ and gates G₁₉, G₂₆, G₃₁, G₂ ' and G₄ ', causing 1/T of AR to be applied to discriminating circuit I through gate G₁₉ and to be again to AR through gate G₃₁. To I is also applied a digital value corresponding to "1" of R₂, thereby the above mentioned discrimination is performed in the discriminating circuit of FIG. 7. When 1/T> 1, F₂ of RSFF produces an output "1", while when 1/T≦ 1, it produces an output "0". Tenth signal S₁₀ is applied to register AR and gates G₂₅, G₂₆ and G₃₁, causing the content of AR to be displayed by display device DS through gates G₂₅ and G₃₃ when 1/T> 1 or through gates G₂₅ and G₃₄ and inverter IN and reciprocal number circuit 6, when 1/T≦ 1 and causing the content of AR to be again introduced to AR through gate G₃₁.

For making an aperture value determination, a desired shutter time is set into the meter by operating certain of digit keys 0 through 9 and a decimal point key of FIG. 3, and then key "T" is pushed down, thereby gate G₇ is gated on, and the desired shutter time is introduced to memory register M3R and stored therein. Next, key A is pushed down to select the aperture value computation program of FIG. 9 for cooperation with ROM device. When signal S₁ occurs, the content of RN part of M1R is compared with "1". When no light measurement is previously made, signals E₁ through E₄ occur, causing Ev value to be introduced to AR. When at least one light measurement was made, signals S₂ and S₃ occur causing 2^(Ev) to be introduced to BR. Next, first signal A₁ is applied to M3R, AR and gates G₁ and G₂₆, causing the content of M3R to be transferred to AR through gate G₁. Second signal A₂ is applied to registers AR and CR and gates G₃ and G₂₈, causing T value to be introduced from AR to CR through gates G₂₆ and G₃. Third signal A is applied to registers BR and CR and gates G₁₆, G₁₇, G₂₀, G₂₆, G₂₇ and G₂₈ causing the outputs of BR and CR to be applied to multiplier 5 through gates G₁₆ and G₁₇, and causing the output of multiplier 5 to be applied through gate G₂₀ to AR. Fourth signal A₄ is applied to register AR, and gates G₈, G₂₀ and G₂₆, causing the output of AR with a magnitude representative of 2^(Ev) × T to be applied through gate G₈ to root circuit 1 and then causing the output of root circuit 1 with a magnitude representative of √2^(Ev) × T to be applied through gate G₂₀ to AR. Fifth signal A₅ is applied to register AR and gates G₂₆, G₃₃ and G₃₁ causing the content of AR which represents the square-root of 2^(Ev) × T(=A) to be displayed. FIG. 10 is a circuit diagram of adder 2 of FIG. 1 of which input terminals are connected to the gates G₉ through G₁₂ of FIG. 1, and which is constructed from NAND gate, AND gates and a 1-bit shift register. FIG. 11 shows an example of the construction of the film speed setter Sv of FIG. 1. The setter Sv has a code plate COD and a number of brushes Br.sub. 0 through Br.sub. 3 arranged in movably contacting relation with the code plate COD at the conductance patterns thereof to constitute the corresponding number of switches SW₁ through SW₄ when switch DIN of FIG. 3 is turned on. Using the code plate COD, a desired film speed can be set into a register RG in the digital form. FIG. 12 is an example of the reciprocal number circuit 6 of FIG. 1 as comprising a divider having two input terminals, one of which is connected to the above mentioned gate G₃₄ and the other of which is connected to the input terminal of a register SR having a digital value corresponding to "1". Responsive to an input from gate G₃₄, the divider produces an output with a magnitude representative of the reciprocal number of the input from gate G₃₄. FIG. 13 shows an example of each of the shift registers AR, BR, CR, M1R, M3R, M4R, R₁ and R₂ of FIG. 1 as comprising four JK flip-flop FF, an inverter IN and an AND gate AND having two input terminals one of which is connected to the clock pulse generator and the other of which is connected to ROM device.

As described above, the digital exposure meter of the invention is constructed from a digital electrical circuits which may be integrated and enables the operator to select various functions of operation, thereby the capabilities of the exposure meter are broadened. 

What is claimed is:
 1. A digital exposure meter comprising: a. a first conversion means for converting the value of the intensity of light into a first digital signal;b. a second conversion means for converting the value of an exposure factor into a second digital signal; c. a plurality of register means, at least two of said register means being connected with each other and each having a first transfer gate through which the content of each said register means is transferred from one to another; d. applying means for applying said first and said second digital signals from said first and second conversion means to said register means; e. a computing circuit, said computing circuit including at least addition-computing means having addition function and division-computing means having division function, and each of said computing means being connected to at least two of said plurality of register means through input gates and having a second transfer gate through which the output of said computing means is transferred to said register means; f. sequence control means connected to said first transfer and said second transfer gates and input gates for generating control signals in sequence which are selectively applied to said gates to control the gating operation of each of said gates; g. regulating means for regulating transmission of said control signals to said gates in predetermined sequential relation; and h. display means connected to at least one of said register means for displaying the content of said register means.
 2. A digital exposure meter comprising:a. a first conversion means for converting the value of the intensity of light into a first digital value proportional to the logarithm of the light intensity; b. a second conversion means for converting the value of film speed to be set into a second digital value proportional to the logarithm of the film speed value; c. adder means connected to said first conversion means and said second conversion means for adding the first digital value and second digital value, thereby said adder means provides an exposure value (Ev value) which is a sum of said first digital value proportional to the logarithm of the light intensity and said second digital value proportional to the logarithm of the film speed value; d. a fourth conversion means for converting the shutter time value (T value) to be set into a fourth digital value; e. exponent means connected to said adder means for producing a 2^(Ev) value based on said exposure value (Ev value) produced by said adder means; f. multiplier means connected to said fourth conversion means and said exponent means for multiplying said 2^(Ev) by said fourth digital value so as to produce a 2^(Ev) × T value; and g. root means connected to said multiplier means for computing the square-root of said 2^(Ev) × T value.
 3. A digital exposure meter comprising:a. a first conversion means for converting the value of the intensity of light to be photographed into a first digital value proportional to the value of light intensity; b. exposure information input means for setting digital values corresponding to exposure informations such as film sensitivity information, diaphragm value information and shutter time information; c. operation mode selecting means for selection of operation modes such as aperture value computing mode, shutter time computing mode, exposure value computing mode and average exposure value computing mode; d. sequence control signal generating means for generating a train of control signals in a manner based on the operation mode selected by said operation mode selecting means; e. an exposure computing circuit connected to said first conversion means, said exposure information input means and said sequence control signal generating means upon response to a control signal from said sequence control signal generating means for performing a computation; and f. display means for displaying the output of said exposure computing circuit.
 4. A digital exposure calculator including:a. keyboard input means having a plurality of numeric keys manually operable for entering numerical data into the calculator and having a plurality of non-numeric control keys manually operable for controlling the calculator, said non-numeric control keys including a plurality of keys for exposure computation mode selection associated with a plurality of exposure computation programs, said exposure computation programs each having a series of computation operations; b. storage means coupled to said keyboard input means for storing numerical data entered into or calculated by the calculator; c. processing means for processing said numerical data in accordance with the exposure computation program selected by said exposure computation keys; and d. display means connected to said storage means for displaying the output of said processing means as a result of computation.
 5. A digital exposure meter comprising:a. a first conversion means for converting the value of the intensity of light into a first digital value proportional to the logarithm of the light intensity; b. a second conversion means for converting the value of film speed to be set into a second digital value proportional to the logarithm of the film speed value; c. adder means connected to said first conversion means and said second conversion means for adding the first digital value and second digital value provides an exposure value (Ev value) which is a sum of said first digital value proportional to the logarithm of the light intensity and said second digital value proportional to the logarithm of the film speed value; d. a third conversion means for converting the aperture value (A value) to be set into a third digital value; e. exponent means connected to said adder means for producing a 2^(Ev) value based on said exposure (Ev) value produced by said adder means; f. divider means connected to said exponent means and said third conversion means for dividing said 2^(Ev) value by said third digital value so as to produce a 2^(Ev) /A² value, whereby the computation of 2^(Ev) /A² = 1/T is resulted.
 6. A digital exposure meter according to claim 5, wherein said divider means is provided with discriminating means for comparing the output value 1/T with 1 and with reciprocal number means responsive to the output of said discriminating means for computing the reciprocal number of 1/T, whereby when 1/T<1, said discriminating means produces an output signal which renders operative said reciprocal number means.
 7. A digital exposure meter comprising:a. a first conversion means for converting the value of the intensity of light into a first digital value proportional to the logarithm of the light intensity; b. a second conversion means for converting the value of film speed to be set into a second digital value proportional to the logarithm of the film speed value; c. adder means connected to said first conversion means and said second conversion means for adding the first digital value and second digital value, thereby said adder means providing an exposure value (Ev value) which is a sum of said first digital value proportional to the logarithm of the light intensity and said second digital value proportional to the logarithm of the film speed value; d. exposure value computation number detecting means for detecting the number of exposure value computations repeated, said detecting means being capable of memorizing the number of exposure values (Ev values) produced from said adder means; e. summing means for summing up all of the Ev values produced from said adder means; and f. divider means connected to said exposure value computation mumber detecting means and to said Ev value summing means for dividing the sum of the Ev values by the number of computations repeated so as to produce an average exposure (Ev) value.
 8. A digital exposure meter according to claim 7, wherein said exposure value computation number detecting means is provided with second adder means for adding a digital value corresponding to 1 each time said adder means produces an Ev value.
 9. A digital exposure meter according to claim 7, wherein said Ev value summing means is provided with third adder means for adding an Ev value produced from said adder means each time the Ev value is produced. 